Conversion and separation of isomeric xylenes



July 29, 1969 G. R. LESTER CONVERSION AND SEPARATION OF ISOMERIC XYLENES Hydrgen Make up Filed Dec. 4. 1967 Mixed Xy/enes //V V EN TOR' George Lesfer md/URW;

TTORNEYS United States Patent O 3,458,589 lCONVERSION AND SEPARATION OF ISOMERIC XYLENES George R. Lester, Park Ridge, Ill., assignor to Universal Oil Products Company, Des Plaines, Ill., a corporation of Delaware Continuation-impart of application Ser. No. 425,834, `lan. 15, 1965. This application Dec. 4, 1967, Ser. No. 687,740 The portion of the term of the patent subsequent to May 21, 1985, has been disclaimed Int. Cl. C07c 15/08, 7/04, 5/10 U.S. Cl. 260-674 6 Claims ABSTRACT F THE DISCLOSURE Related applications The present application is a continuation-in-part of my copending application Ser. No. 425,834, led Jan. 15, 1965 now U.S. Patent No. 3,384,676, issued May 21, 1968, all the teachings of which patent are incorporated herein by Way of specific reference thereto.

Applicability of invention Although the present process, encompassing the inventive concept described herein, is applicable to the preparation of naphthenic hydrocarbons via hydrogenation of aromatics, it is specically directed toward the hydrogenation of Ca-aromatic hydrocarbons. The process is especially adaptable for effecting the hydrogenation of an aromatic hydrocarbon mixture containing meta-xylene and para-xylene, to yield a dimethylcyclohexane product efuent, the geometric isomer distribution of which is predominantly in the cis form. As such, the hydrogenated product is readily separable by ordinary fractionating/ distillation means as distinguished from the so-called super-fractionators. It will be easily recognized by those cognizant of petroleum processing operations and rening techniques, that the present invention may be advantageously integrated with a catalytic reforming process Wherein significant quantities of Ca-aromatic hydrocarbons are produced from a parain-naphthene charge stock. Many such catalytic reforming processes are currently designed and operated for the specific purpose of maximizing both the yield of Cra-C4 LPG (liquied petroleum gas) and of aromatic hydrocarbons.

Within the classication of C-arolmatic hydrocarbons, there are four principal isomers; ethylbenzene, ortho-xylene, para-xylene and meta-xylene. With the exception of the latter, all experience widespread utility and high commercial value, for example in the production of styrene, phthalic acid and terephthalic acid, respectively. Regardless of the particular Cs-aromatic, or its intended use, economic considerations dictate that the C-aromatic be available in `a substantially pure state, and especially substantially free from its isomers. In general, abundant Patented July 29, 1969 'ice quantities of C-aromatics are separated from various product streams by way of a suitable solvent extraction technique. Thus, for example, a typical C-aromatic fraction separated from a catalytically reformed product eflluent, contains from 8-14% ethylbenzene, 18-24% ortho-xylene, 17-21% para-xylene and 45-53% metaxylene. As such, the fraction may be utilized in a comparatively low value service, either as a gasoline blending component, or Ias a solvent. In order, however, to utilize the Cg-aromatic fraction in a relatively high value service, separation into substantially pure component streams is required. Exemplary of the uses of meta-xylene include its utilization `as a solvent, in insecticides, as an intermediate in the preparation of dyes and in organic synthesis such as in the preparation of isophthalic anhydride. Orthoxylene is most often used in the preparation of phthalic anhydride, but is employed to some extent in the pharmaceutical industry. Para-xylene is probably the most valuable the xylene isomers in view of the great extent to which it is used in the preparation of terephthalic acid, as well l as for the production of synthetic resins and fibers.

Ortho-xylene is readily separated from the Cs-arolmatic mixture through the use of ordinary fractionation facilities. While it is somewhat more difficult to separate ethylbenzene from the remainder of the mixture, this is being currently accomplished on a commercial scale. However, it is not economically feasible to separate metaand para-xylene into substantially pure streams by way of ordinary, direct fractionation. Since their normal boiling points are so close, it would necessitate the use of an immense fractionator, having many hundreds of trays, to achieve the separation. Other methods have been suggested in the past, including fractional crystallization. However, these involve the handling of solids and the use of subatmospheric temperatures; in general, such methods are prohibitively expensive.

Prior art As hereinbefore stated, the separation of para-xylene from a mixture thereof with meta-xylene is not economical through the use of ordinary fractionation due to the close proximity of their normal boiling points; metaxylene boils at 139.3 C., while the boiling point of paraxylene is 138.5 C. Candor compels recognition of the fact that the `broad concept of hydrogenating a Cg-aromatic mixture has been considered by the prior art. Although fractionation of the resulting naphthenic mixture has provided a substantially pure ethylcyclohexane stream, further separation has not been .attempted due to the similarity in boiling points of the equilibrium geometric isomer distribution of 1,3-dimethylcyclohexane and 1,4- dimethylcyclohexane. In United States Patent No. 2,282,231, Mattox recognized the concept of hydrogenating a C-aromatic mixture, distilling to separate ethylcyclohexane rand indicating only 1.0 C. difference between the boiling points of 1,3- and 1,4-dimethylcycl0- hexane. That is, Mattox was primarily concerned only with the recovery of ethylbenzene by separation from a mixture thereof with mixed xylene isomers, and there is no recognition of, or even concern for, the recovery of substantially pure isomeric xylenes. Furthermore, the catalyst employed in the hydrogenation step of this prior art process is shown as producing mixed 1,3- and 1,4- Idimethylcyclohexanes having a boiling point difference of only one degree centigrade. As hereinafter indicated, there exists two geometric isomeric forms of both 1,3- -and 1,4-dimethylcyclohexane, the cis form and the trans form. Through the use of the present invention, the hydrogenated product predominates in the cis forms which have a boiling point differential of slightly more than 4.0 C.

Objects and embodiments A principal object of the present invention is to form an isomeric mixture of 1,3-dimethylcyclohexane and 1,4- dimethylcyclohexane which predominates in the cis form of the isomers. A corollary objective resides in hydrogenating a mixture of metaand para-xylene while preventing equilibrium geometric isomer distribution within the hydrogenated product.

Another object is to separate a mixture of meta-xylene and para-xylene into substantially pure component streams without resort to super-fractionation equipment. In conjunction, the object is to recover para-xylene from a mixture of C-aromatics in an economical manner.

Therefore, in a broad embodiment, the present invention encompasses a process which comprises hydrogenating a mixture of meta-xylene and para-xylene in contact with a non-acidic hydrogenation catalyst, to form an isomeric mixture of 1,3-dimethylcyclohexane and 1,4-dimethylcyclohexane, said mixture predominating in the cis form of the isomers, and fractionating said isomeric mixture to provide a predominantly cis 1,3-dimethylcyclohexane fraction and a predominantly cis 1,4dimethylcyclohexane fraction.

In another broad embodiment, the present invention relates to a process for separating para-xylene from metaxylene, which process comprises the steps of: (a) reacting a mixture of para-xylene and meta-xylene with hydrogen, in contact with a non-acidic hydrogenation catalyst, at hydrogenating conditions selected to produce 1,3- and 1,ll-dimethylcyclohexanes, said 1,4-dimethylcyclohexane being in a greater concentration of the cis geometric isomer than equilibrium; (b) separating the 1, 3-dimethylcyclohexane product from 1,4-dimethylcyclohexane product by ordinary fractionating-distillation means; and, (c) dehydrogenating said 1,4-dimethylcyclohexane product at conditions selected to produce paraxylene.

Other objects and embodiments will become evident from the following description of the present invention and the preferred means for effecting the same.

Summary of invention As hereinbefore set forth, the prior art indicates a one degree centigrade diiference in the normal boiling points of 1,3- and 1,4-dimethylcyclohexane and, therefore, there is no recognition of the fact that these naphthenes can be utilized as intermediate component products in the separation of mixed xylenes, and especially in the separation of para-xylene from a mixture thereof with meta-xylene. However, there are two geometric isomeric forms of both the 1,3- and 1,4-dimethylcyclohexane, the cis and the trans form; these geometric isomers have the normal boiling points indicated in the following table:

TABLE: ISOMERIC DIMETHY-LCYCLOHl-EXANEl BOILING POINTS Compound: Boiling point, C. cis 1,3-dimethylcyclohexane 120.09 trans 1,3-dimethylcyclohexane 124.45 cis 1,4-dimethylcyclohexane 124.32 trans 1,4-dimethylcyclohexane 119.35

It will be noted that there is slightly more than a 4 C. difference in boiling points between the cis isomers of the 1,3- and 1,4-dimethylcyclohexanes; this difference is sufhcient to allow the economical separation of these geometric isomers by direct fractionation. However, the equillibrium distribution of the isomers is such that the average boiling point of the equilibrium mixture of the 1,3-dimethylcyclohexane isomers is very close to the average boiling point of the equilibrium mixture of the 1,4-dimethylcyclohexane isomers and accordingly the 1, 3-compounds cannot be economically separated from the 1,4-compounds by ordinary direct fractionation.

I have observed that, in the hydrogenation of both paraand meta-Xylene, the cis form predominates as the direct or kinetic hydrogenation product. In the case of the 1,3-dimethylcyclohexane, the cis form is also predominant at equilibrium under hydrogenation conditions, Whereas in the case of the 1,4-dimethylcyclohexane the trans form is predominant at said equilibrium. Therefore, after the xylenes have been hydrogenated, the 1,3- dimethylcyclohexane tends to remain in the cis form whereas the 1,4-dimethylcyclohexane tends to be converted by geometric isomerization from the cis form to the trans form. Such conversion is undesirable since the boiling points of the 1,3-isomer and the 1,4-isomer tend to approach each other thus rendering separation by ordinary fractionation difficult if not impossible. Furthermore, the reaction rate of geometric isomerization is accelerated through the use of an acidic catalyst. It is wellknown that hydrogenation catalysts having a number of acid sites catalyze isomerization reactions. I have found that the use of a hydrogenation catalyst having a few or preferably no acid sites, causes the geometric isomerization reaction to be minimized, thus maintaining the predominantly cis form of both the 1,3- and 1,4- dimethylcyclohexanes. Furthermore since equilibrium favors the cis form of the I1,3-isomer, there is little tendency for it to convert to the trans form, whereas whatever small amount of the cis form of the 1,4-isomer converts to the trans form will have a boiling point very close to the cis 1,3-isomer and therefore a stream of high purity cis 1,4-isomer can be withdrawn as a bottoms product from an ordinary fractionator. Accordingly, through the use of a non-'acid hydrogenation catalyst in the hydrogenation of metaand para-xylene, the cis form of 1,3- and 1,4-dimethylcyclohexanes are the predominant products with only a small quantity, if any, of trans 1,4-dimethylcyclohexane. When this hydrogenated product is fractionated, a stream of cis 1,3-dimethylcyclohexane and perhaps a small amount of trans 1,4-dimethylcyclohexane is removed overhead while a concentrated cis 1,4-dimethylcyclohexane stream is removed from the bottom of the fractionator. The bottom stream may be dehydrogenated in contact with a non-acid dehydrogenation catalyst to produce a substantially pure para-xylene stream. The hydrogen evolved during the dehydrogenation step may be recycled to the hydrogenation reaction zone, further enhancing the economics of the present process.

Description of drawings My invention, and its use as a valuable tool in petroleum rening processes, can be clearly understood by reference to the acompanying figure. ln the drawing, illustrative of the particular embodiment, various heaters, coolers, compressors, controllers, instruments, valves, start-up lines, etc., have been eliminated or reduced in number as not being essential to a clear understanding of my inventive concept. These, as well as other miscel- Ianeous appurtenances are well within the purview of those skilled in the art of petroleum rening processes and techniques.

With reference now to the drawing, a simplified ow scheme wherein the mixture of metaand para-xylene is introduced via line 1, is represented. The xylenes are admixed with hydrogen flowing in line 2, and the xylene/ hydrogen stream continues through line 1 to be introduced into hydrogenation reactor 3. The hydrogenated product effluent is withdrawn by way of line 4, and is passed thereby into high-pressure separator 5. A normally gaseous phase is removed through line 6 by compressive means not illustrated, and is recycled, by way of line 2 to combine with fresh Xylene feed. Make-up hydrogen, to replace that which is consumed in the hydrogenation reactor, is introduced into the system through line 15.

The normally liquid hydrogenated product effluent, comprising principally the cis forms of 1,3- and 1,4-dimethylcyclohexane, is withdrawn from separator 5 through line 7, and is introduced thereby into fractionator 8. An overhead stream, consisting substantially of 1,3-dimethylcyclohexane, is removed as an overhead fraction through line 10. The remaining net overhead product can be advantageously utilized in many ways. For example, it may be dehydrogenated to a substantially pure meta-xylene product; it may be introduced into a catalytic reforming unit, as recycle, or charge, since it is substantially naphthenic. The latter is particularly desirable in view of the rapidity with which the dehydrogenation reaction is effected during catalytic reforming. The concentration of hydrogen in the reforming reaction zone increases, resulting in enhanced reforming catalyst stability. Since the recycled overhead is cornposed predominantly of the meta (1,3) form, and the reforming catalyst promotes isomerization reactions, a portion of the 1,3 material Iwill be isomerized to produce the equilibrium isomer distribution of ortho-xylene, paraxylene, ethylbenzene and meta-xylene. As the overall effect, desirable ortho-xylene, para-xylene and ethylbenzene will be produced as net products, while meta-xylene will be thereby recycled to extinction.

The bottoms product from fractionator 8, comprising substantially pure cis 1,4-dimethylcyclohexane, is withdrawn through line 19. Not indicated in the drawing is the diversion of a portion of the line 19 product through a reboiler heater accompanied by re-introduction of the heated portion into fractionator 8 to maintain the desired bottoms temperature. The remaining net bottoms product continues through line 9, and is introduced thereby into dehydrogenation reaction zone 11. The dehydrogenated product, the normally liquid portion of which is substantially pure para-xylene, is withdrawn through line 12 into separator 13. The net gas from separator 13 ows through line 2 as an additional supply of hydrogen for hydrogenation reactor 3. The normally liquid effluent leaves separator 13 through line 14, and comprises substantially pure para-xylene. Since the make-up gas .owing into the process through llow conduit 1S generally contains some light hydrocarbons, at least a portion of the separator gas in line 6 is vented so that the hydrogen purity of the recycle gas will remain high.

The essence of this invention resides in the use of a hydrogenation step in which the kinetic reaction products are preserved while geometric isomerization reactions are avoided. The catalyst disposed in reactor 3 is, therefore, a composite containing few acid sites and preferably one which has no acid activity whatsoever. A preferable hydrogenation catalyst comprises an alumina support which has been treated with an alkali metal cation solution to neutralize acid sites, and having a Group VIII metal impregnated thereon. Especially preferable alkali metals are lithium, sodium and potassium while especially preferable Group VIII metals comprise platinum and nickel. Other non-acid supports may be utilized instead of alumina. Platinum is a very active hydrogenation metal and is incorporated on the catalyst in concentrations of from about 0.2 weight percent to about 2.0 weight percent and preferably about 0.75 weight percent. On the other hand, nickel is a less active hydrogenation metal and is incorporated on the catalyst support in concentrations of from about 1 weight percent to about 50 weight percent and preferably about 25 weight percent. The alkali metal concentration should be sufficient to neutralize all the acidity on the finished catalyst, and concentrations of from about 0.01 weight percent to about 1 weight percent are generally sufficient, although concentrations of about 0.2 to about 0.5 weight percent are preferred. The operating conditions utilized in hydrogenation reactor 3 include pressures of from about 200 p.s.i.g. to about 800 p.s.i.g. and preferably from 300 to 500 p.s.i.g., temperatures of from about 75 F. to about 6 800 F., and preferably from about 200 F. to about 400 F., liquid hourly space velocities of about 0.1 to aboutl 10.0 and preferably about 0.5 to about 5.0, and hydrogen to oil molal ratios of about 2 to about 20 and preferably about 3 to about 10.

Fractionator 8 may function at any convenient pressure, although low pressures are preferably employed to accentuate the differences between the boiling points of the 1,3- and l,4-dimethylcyclohexanes. Since a large IJfractionator must be utilized to make the above split, low pressure drop stages such as low pressure drop trays should be employed in order to achieve an economical separation.

Thevdehydrogenation reactor 11 contains the alumina, alkali metal, Group VIII metal catalyst described hereinbefore in order to prevent any isomerization to the Cs-isomers, when dehydrogenating the cis 1,4-dimethylcyclohexane to para-xylene. The operating conditions utilized inthe dehydrogenating reactor are pressures of from about 25 to about 500 p.s.i.g. and pre-ferably from 50 to 300 p.s.i.g., temperatures of from about 100 F. to about 900 F. and preferably Ifrom about 250 F. to about 450 F., liquid hourly space velocities of about 0.1 to about 20.0 and preferably within the range of from about 0.5 to about 10.0, and hydrogen to oil molal ratios of about 2 to about 20.

With respect further to the conditions under which dehydrogenation zone 11 and hydrogenation zone 3 are maintained, the latter will function at a greater pressure and lower liquid hourly space velocity than the former. The hydrogen to hydrocarbon molal ratios iwill generally be the same, although this is not an essential consideration. However, in order to strike an optimum economic balance between the cis form of 1,4-dimethylcyclohexane produced via hydrogenation, and the selectivity of the dehydrogenation thereof to para-xylene, the hydrogenation conditions include a temperature of at least 50 F. less than the temperature at which the dehydrogenation is effected.

EXAMPLES The following examples are inclined to illustrate the novelty and utility of the present invention, but are not intended to limit the invention to the conditions or materials shown therein.

Example I This example is presented to illustrate that the utilization of an alkali metal, non-acid hydrogenation catalyst, -to hydrogenate para-xylene, will result in a greater concentration of the cis 1,4 dimethylcyclohexane than equilibrium geometric isomer distribution. A hydrogenation catalyst having an 0.54 ABD (apparent bulk density) is prepared having an alumina carrier material, 0.75 weight percent platinum and 0.33 weight percent lithium, calculated as the elements. Twenty cc. of the above described catalyst is placed into a glass lined autoclave, and cc. of para-xylene is added. The contents are pressured up to 200 atmospheres with hydrogen. The autoclave and contents are heated to ya temperature of 392 F. and maintained at that temperature for a period of one hour at the end of which time the autoclave and contents thereof are cooled to room temperature and a sample of the liquid in the liner is taken and analyzed by gas chromatography. The liquid sample contained 35.2 weight percent trans 1,4 dimethylcyclohexane, 44.0 weight percent cis 1,4 dimethylcyclohexane, 20.5 weight percent para xylene and trace quantities of other xylenes. At these conditions, the equilibrium geometric isomer distribution of the cis -to the trans 1,4 dimethylcyclohexanes is about 25 to 75. It should be noted that the concentration of cis 1,4 dimethylcyclohexane is considerably higher than equilibrium calls for, and in a continuous process in which the residence time in the reactor is considerably less than 1 hour it is expected that even higher concentrations of the cis 1,4 dimethylcyclohexanes are produced.

Example II A charge stock containing 65 weight percent metaxylene and 35 weight percent para-xylene is introduced into a fixed bed of the catalyst described in Example I. This first reactor is surrounded by three block type heater such that the reactor is maintained at substantially isothermal conditions. The temperature is maintained at 392 F. and the pressure is maintained at 500 p.s.i.g. Substantially pure hydrogen is introduced into the reactor on pressure control and the separator gas is recycled back to thereactor at such a rate as to maintain a hydrogen to oil mole ratio of 8.0. The normally liquid reactor efuent is withdrawn from the eiiiuent high pressure, -separator and introduced into a first fractionator wherein a portion of the hydrogenated product containing primarily cis 1,3 dimethylcyclohexane is removed overhead. The bottom stream from the irst fractionator is sent to a second fractionator to remove the unconverted xylenes while also producing a substantially pure cis 1,4 dimethylcyclohexane stream. The latter mentioned stream is introduced into a second reactor containing a fixed bed of the catalyst described in EX- ample I. The second reactor is maintained at a temperature of 752 F. and a pressure of 300 p.s.i.g. The normally liquid second reactor eliluent is withdrawn from the second reactor efiiuent separator and, after removing the unconverted naphthenes, comprises a xylene stream containing para-xylene in excess of 95 weight percent.

Example III This example is included to illustrate the improved reforming operation when charging a feed of enriched naphthene content as compared to a conventional naphtha. A fixed-bed reactor containing a catalytic composite of 0.375 weight percent platinum and 0.9 weight percent chloride on an alumina carrier material, is utilized. A naphtha charge stock having a 170 F. initial boiling point and a 300 F. end point, by Engler distillation, a parain content of 55 volume percent, a naphthene content of 35 volume percent and an aromatic content of 10 volume percent is introduced into the reforming reactor at a liquid hourly space velocity of 1.4, a pressure of 300 p.s.i.g. and a hydrogen to hydrocarbon mole ratio of 7. The temperature in the reactor is adjusted to maintain the octane number of the C5+ reformate at 95.0 F-l Clear.

Another run is subsequently made on a fresh batch of the same reforming catalyst at the same conditions as above utilizing a second different charge stock. The Engler distillation of this second stock indicates the same initial and end boiling points, but a hydrocarbon type analysis indicates 45 volume percent paraiiins, 45 volume percent naphthenes and volume percent aromatics. The second charge stock is introduced into the reactor and the temperatures are adjusted to maintain the octane number of the C5+ reformate at 95.0 F-l Clear.

The results of the run with the lirst charge stock is compared with the result of the run with the second stock and the following conclusions are observed. The

run with the second charge stock had about a 25% greater catalyst stability, an increased C5+ reformate yield of about 3.5 volume percent, an increased separator gas yield of about standard cubic feet per barrel of charge and an increase in hydrogen purity in the recycle gas of about 5 mole percent. These results clearly indicate the desirable effect a charge stock of enriched naphthene content has upon a reforming process.

The foregoing specification and examples indicate the method of effecting the present invention, and the benefits to be aior-ded through the utilization thereof.

I claim as my invention:

1. A process for separating para-xylene from a mixture thereof with meta-xylene which comprises the steps of:

(a) reacting a mixture of para-xylene and meta-xylene with hydrogen, in contact with a nonacidic hydrogenation catalyst, at conditions selected to produce 1,3 dimethylcyclohexane and 1,4 dimethylcyclohexane, the latter having a greater concentration of the cis geometric isomer than equilibrium;

(b) separating the resulting hydrogenated product by ordinary fractionating-distillation means to provide a 1,3-dimethylcyc1ohexane fraction and a substantially pure 1,4-dimethylcyclohexane fraction;

(c) dehydrogenating said 1,4 dimethylcyclohexane fraction in contact with a dehydrogenation catalyst and at dehydrogenating conditions; and,

(d) separating the resulting dehydrogenated product to provide a hydrogen-rich gaseous phase and a substantially pure para-xylene liquid phase, and recovering the latter.

2. The process of claim 1 further characterized in that said hydrogenation catalyst is a composite of alumina, a `Group VIII metallic component and a metallic component selected from the group consisting of lithium, sodium and potassium.

3. The process of claim 2 further characterized in that said hydrogenation catalyst is a composite of alumina, platinum and lithium.

4. The process of claim 2 further characterized in that said hydrogenation catalyst is a composite of alumina, nickel Iand lithium.

5. The process of claim 1 further charatcerized in that said hydrogenating conditions include a temperature at least about 50 F. less than the temperature at which said 1,4-dimethylcyclohexane is subjected to dehydrogenation.

6. The process of claim 1 further characterized in that said hydrogen-rich gaseous phase is recycled to combine with the aforesaid mixture of paraand meta-Xylene.

References Cited UNITED STATES PATENTS 2,282,231 5/ 1942 Mattox 260-674 2,920,114 1/ 1960 Bloch 260-668 3,113,978 12/1963 Dertig et al. 260-674 DELBERT E. GANTZ, Primary Examiner C. E. SPRESSER, Assistant Examiner Us. c1. xn. 

